Vibrations of Monosubstituted Octasilasesquioxanes

Abstract

The vibrational spectra of monosubstituted spherosiloxanes of the type RH7Si8O12 can be understood as a superposition of the spectral features of H7Si8O12 and of the substituent R. We show this for the FT-Raman spectra of RH7Si8O12 with R = -CH2-CH2- C6H5 and -CH = CH-C6H5. The force fields of H8Si8O12 and of the organic substituent, ethylbenzene and styrene, respectively, were combined and normal coordinate analysis was applied. The spectra of phenethyl-H7Si8O12 and styryl-H7Si8O12 were correlated with the spectrum of H8Si8O12. The Si-X stretching frequencies in the spherosiloxanes are larger than in most other siloxane compounds. We have shown that this is due to the different X-Si-O-Si conformations, which are anti and syn, respectively, and can be explained by the dependence of the bond order on the X-Si-O-Si dihedral angle.